Integrated electrical circuits and other microstructured components are usually produced by virtue of a plurality of structured layers being applied onto a suitable substrate, which is usually a silicon wafer. For the purposes of structuring the layers, these are initially covered by a photoresist (resist), which is sensitive to light from a specific wavelength range, e.g. light in the deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) spectral range. Subsequently, the wafer thus coated is exposed in a projection exposure apparatus. Here, a pattern of diffractive structures, which is arranged on a mask, is imaged onto the photoresist with the aid of a projection lens. Since the absolute value of the imaging scale generally is less than 1 in this case, such projection lenses are sometimes also referred to as reduction lenses.
The wafer is subjected to an etching process after developing the photoresist, as a result of which the layer is structured in accordance with the pattern on the mask. The photoresist which still remained is then removed from the remaining parts of the layer. This process is repeated until all layers are applied to the wafer.
References disclose illumination systems which use mirror arrays in order to be able to variably illuminate the pupil plane of the illumination system. Examples are found in EP 1 262 836 A1, US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1. In general, these are mirror arrays in which the mirrors can be tilted continuously over a certain angle range.
WO 2012/100791 A1 disclosesan illumination system which additionally includes a digitally switchable micromirror array. This micromirror array is imaged onto the light-entrance facets of an optical integrator with the aid of a lens. A similar illumination system, albeit with a different actuation, is known from the European patent application, filed on Nov. 22, 2013, with the file reference EP 13194135.3 and the title “Illumination System of a Microlithgraphic Projection Exposure Apparatus”, the contents of which is hereby incorporated by reference.
When digitally switchable micromirror arrays are used in illumination systems, the number of micromirrors involved is generally so high that it cannot be provided by a single micromechanical component. It is for this reason that the previously mentioned European patent application with the file reference EP 13194135.3 has proposed arranging a plurality of relatively small micromirror arrays next to one another on a carrier and imaging these onto the target surface with the aid of a special imaging optical unit in such a way that the images of the individual arrays adjoin one another seamlessly on the target surface.
Similar problems arise if the array in the illumination system does not contain any micromirrors but optical elements which are switchable in a different manner, e.g. liquid crystal cells, as are used in LCDs.
A further problem when using arrays with switchable optical elements consists of the fact that, occasionally, the optical elements are spaced relatively far apart. If such an array is imaged onto the target surface, correspondingly large gaps arise between the images of the optical elements, which is often undesirable.
U.S. Pat. No. 6,624,880 B2, US 2011/0134407 A1 and U.S. Pat. No. 7,957,055 B2 disclose projection exposure apparatuses, in which an illumination system does not illuminate any conventional masks but a micromirror array which assumes the function of the mask. Such apparatuses are often also referred to as “mask-less”. US 2011/0240611 A1 describes a projection exposure apparatus, in which a one-dimensional micromirror array is used to directly ablate material on the wafer. US 2012/0081685 A1 has disclosed an illumination system in which the polarization direction of the light can be switched over between two successive light pulses.